IPTV isn't on checked this earlier and was going to turn it on and off but turning it on takes a few steps so didn't even try that as thought it was so unlikely.

@Alexandre. Hi I haven't done that but swapping the main unit for one of the others and asssigning it as the main one makes me think it likely isn't an issue with the port but a setting or firmware...

quick update to this I have swapped the main deco unit for one of the others and set it as the main and seem to be having the same issue so it must be a setting or firmware issue

I have had 3 Deco X20 units set up and working fine for months but in the last week on 2 occassions the 2nd ethernet port on my main unit has seemed to stop working. I have the setup as per the image...